44 PRACTICAL BOTANY. 



87. The sbmcbwre of the Ifeaf presents (at least ia the 

 higher organized plants) two parts — woody tissue and j>a- 

 renchyma. "While the former, derived from the liber, and 

 composing the framework, exists in small quantity, the lat- 

 ter, the green pulp, is predominant, and is nearly identical 

 with the green layer of the bark. The cells of the green 

 pulp exist in two layers, beneath the epidermis. The 

 upper layer consists of oblong cells, placed perpendi- 

 cularly to the surface of the leaf ; the lower is made \ip of 

 similar cells placed parallel to the same, and traversed by 

 intercellular passages, so as to be less compact than the 

 upper layer. 



The green color of the leaf is owing to a green pigment, 

 lying loose in its cells, and called chlorophyll. 



Not only the JPhcBnogams, but also the higher-grade 

 Cryptogams, have leaves, consisting of both wood-fibres 

 and parenchyma. The leaves of the cellular Cryptogams, 

 however, consist of parenchyma alone. 



88. As regards their form,s, we divide the leaves into 

 OEDiNAKY LEAVES ou the One hand, and modified, teams. 

 roKMED, and degeneeated leaves on the other, such as 

 stipules, hraots, irwolucral lea/ves, spathes, scales, pales 

 cupules, hollow leaves, etc. Modified, etc., leaves exhibit 

 essentially the same structure as the ordinary sort. 



* OEDINARY LEAVES. 



80. Ordinary leaves — that is, those which serve par- 

 ticularly as foliage — are divided into, 1, simple, and 2, com^ 

 pound leaves. 



